Cementation Process.

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composition, it may be made either from wrought or cast iron.
We shall first consider the former method.

Cementation.—In this, the oldest process, bars of wrought
iron are placed in fire-clay boxes, Pig. 88, with charcoal dust
around and between them, and a layer of fire-clay over all
(being the cement giving the name to the process). They are then
subjected to a bright-red heat, for a time varying with the amount
of carbon required to be introduced, and which may be as much
as a fortnight for the more highly carbonised steels. The charcoal
then becomes combined with the iron, and the steel so pro-
duced is called blister steel, from the fact that the bars are covered
with blisters. These bars are next broken up, piled, and heated
in a furnace almost exactly like the one in Fig. 85, hammered by
rapid blows from a tilt-hammer, Fig. 89, and shear steel of a

fibrous quality is thus produced.* Double shear steel is made by
breaking in two and re-hammering Crucible cast steel is obtained
by melting fragments of blister steel in covered crucibles made of
a mixture of fire-clay and plumbago, and placed in sets of six or
twelve in furnaces having a similar section to the one shewn in

* The steam hammer is used in later-built works. For drawing, see
Chapter IV.